Hundreds of child welfare professionals, including police officers, care workers and teachers, have been identified as 'extremely high-risk' paedophiles by an investigation into internet porn.

The discovery came after US authorities passed on more than 7,000 names of UK subscribers to an American-based child porn website. When police examined a sample of the most dedicated users, they discovered that many worked with children.

Investigators knew paedophiles targeted jobs which brought them into contact with children, but were shocked by how many British suspects had been undetected by the usual checks.

The discovery that many were working in jobs of the highest sensitivity will send shock waves through the child protection world and lead to calls for even more stringent safeguards.

Investigators now believe as many as 90 police officers have so far been identified from an initial trawl of 200 of the British names found in the US. Many of the other suspects work in other sensitive professions, often linked to the criminal justice system.

John Carr of the Children's Charities' Coalition for Internet Safety (CHIS), which represents all major children's organisations, including the NSPCC, NCH and Barnardos, said: 'It's very distressing to learn that so many policeman are being arrested for these sorts of offences. It undermines people's confidence in our ability to deal with it at all.'

Harry Fletcher, deputy general secretary of the probation union Napo, said: 'There has been growing evidence over the past 10 years that paedophiles have infiltrated careers that put them in direct contact with children. This latest evidence suggests we have to be even more vigilant.'

Specialist child porn investigators, including officers from the National Hi-Tech Crime Unit, have initially concentrated on users who had made more than 10 purchases of child porn from the internet. They used a US web 'portal' that gave access to dozens of sites with titles such as Cyber Lolita, I Am Fourteen and Child Rape. The sites were run by porn barons in Russia and Indonesia and involved abuse committed on captive children.

British involvement in the child porn ring was so large that police set up Operation Ore to investigate all 7,200 subscribers in this country. They have passed on all the names to the relevant forces for further investigation; 700 are known to be in Scotland and 279 in Cambridgeshire. Police believe at least 5,000 could be considered 'goers' and lead to arrests.

Operation Ore has also uncovered civil servants, a firefighter and a teacher in Northern Ireland. Another teacher has been arrested in Cambridgeshire and suspended from working with 11-year-olds at a village school.

One Home Office insider told The Observer that investigations targeted the most serious cases, where arrests and convictions were thought to be relatively straightforward. These involve images of pre-pubescent children where suspects cannot claim they believed they were purchasing adult porn.

The US investigation, Operation Avalanche, was set up by the United States Postal Inspection Service, which probes online porn. It began in Texas and has generated more than 75,000 identities worldwide.

The website portal was run by Thomas and Janice Reedy, a married couple in Fort Worth, Texas, who were running an online porn empire from their suburban mansion which generated £1 million a month in fees. Both have been sent to prison.

The Association of Chief Police Officers (Apco) has warned Ministers that forces do not have the resources to deal with the problem. It is awaiting a Home Office decision on a submission for £2 million of crisis funding.

A Home Office spokeswoman said Ministers welcomed Acpo's focus on the issue, but they had already provided an extra £25 million to help combat hi-tech crime, including child porn.

A police spokesman said: 'There are people up and down this country who are sweating. Give us their computer, even if it's been thrown on a skip, and we can quickly nail them.'